Shigella is an enteric pathogen that causes dysentery in humans. The pathogen is most often acquired by ingestion of contaminated food or water. The annual number of Shigella episodes throughout the world is thought to exceed 150 million. Greater than 90% of the cases occur in developing third world countries. The number of deaths attributed to Shigella exceeds one million per year. The majority of the casualties (>65%) are children under the age of five. Although the mechanism by which Shigella species cause the disease is still not fully understood, the strong inflammatory reaction characteristic to Shigellosis provides a strong indication that host inflammatory cells play a key role relative to the onset of the disease. We hypothesize that Shigella has developed strategies to kills host macrophages and neutrophils in order to survive its first encounter with the host immune surveillance system. The overall goal of studies described in this application is to develop an understanding of the mechanism by which Shigella kills host, human macrophage. Data obtained by the applicants strongly argue that Shigella triggers a non-apoptotic death of human macrophages while Shigella-infected human monocytes die by an accelerated apoptotic process. The studies proposed will test the hypotheses that Shigella-infected macrophages die by a necrotic/non-apoptotic mechanism that i) results from the production of reactive oxygen intermediates that cause the destruction of the mitochondria and/or ii) that a protein produced by virulent Shigella specifically targets the host-cell's mitochondria for destruction. The studies proposed will address two specific aims. Aim 1: Is the non-apoptotic death of human macrophages triggered by virulent Shigella linked to, and dependent on, the production of reactive oxygen intermediates by the infected cells? Aim 2: Is the response of human macrophages to Shigella dependent on the Shigella protein IpgBl?